The present invention relates to an image reading device. More particularly, in an image reading apparatus such as a scanner for use in an image forming apparatus, e.g., an electronic copying machine, the present invention relates to an image reading device and method for irradiating an original put on an original mounting base with a light ray from a light source which moves along the original mounting base and outputting an electrical signal according to a reflected light ray.
Conventionally, in an image reading device such as a scanner for use in an image forming apparatus such as an electronic copying machine, a scanner has been put into practical use, in which an original put on an original mounting base consisting of transparent glass is exposed by an exposure lamp, and a reflected light ray is taken into a CCD line sensor where the light ray is subjected to photoelectric conversion, thereby reading an image on the original.
A mirror or the like for leading the reflected light ray from the exposure lamp or the original to the CCD line sensor is provided on a scanning carriage.
Further, in such an image reading device, for example, a scanner, when reading the original on the original mounting base, an image on the entire original is read by reading that image every line in the main scanning direction while moving the scanning carriage in the sub scanning direction.
Furthermore, in such an image reading device, e.g., a scanner, the light source is moved from a home position to a reading area of the original on the original mounting base in the sub scanning direction of the original by the scanning carriage.
In this case, as shown in FIG. 5A, after being linearly accelerated from the home position by a motor for driving the scanning carriage until a predetermined moving velocity according to a reading magnification is achieved, the scanning carriage passes a reading start position (light irradiation start position) of the image of the original on the original mounting base and performs reading the image of the original on the original mounting base while maintaining a uniform velocity.
In general, as this motor for driving the scanning carriage, a stepping motor driven by pulse control is used.
FIG. 5B shows the state in which a motor electric current is controlled to be switched from a high electric current value to a low electric current value, electric currents of which have the same constant electric current value with any reading magnification, in case of performing linear acceleration and maintaining a uniform velocity by the motor for driving the scanning carriage.
That is, when linear acceleration by the motor for driving the scanning carriage mentioned above is used, since this is simple linear acceleration such that the electric current is controlled to be switched from a high electric current value to a low electric current value, electric currents of which have the same constant values, with any reading magnification, a capacity of firmware including a motor driver and others can be minimum.
Thus, when the output torque is decreased when the velocity (frequency) is increased because of the torque-frequency characteristic such as shown in FIG. 6, a motor which does not step out in a high-velocity domain is required as the motor mentioned above.
That is, in a drive system having a necessary torque domain A such as shown in FIG. 7, since step out of the motor 1 having the illustrated torque-frequency characteristic B is observed, a motor 2 having a torque-frequency characteristic C as shown in the drawing is used.
Here, a difference between the motor 1 and the motor 2 is whether the output torque is increased by extending a rotor length or the like inside the motor.
Therefore, using the motor having the output torque increased in this manner increases the size of the motor itself, which leads to increase in cost.
Furthermore, when the motor having the output torque which is not very large is used, the excessive torque is observed in driving at a low speed even if the electric current value is decreased, which can be a factor for generating undesired vibrations in the scanning carriage.
That is, when vibrations are generated in the scanning carriage, there occurs a serious problem that image vibrations are generated in a result of reading as a scanner due to remaining vibrations.
As mentioned above, in the image reading device such as a scanner, driving the scanning carriage (optical system) using the stepping motor driven by pulse control causes optical reading of the original put on the original mounting base.
Moreover, as a method for controlling acceleration drive of the stepping motor in the image reading device such as a scanner, a control method such as linear acceleration, multi-stage linear acceleration or the like is used to carry out acceleration control in the prior art.
In regard to the method for performing acceleration control, there is known a control method for reducing the remaining vibrations when shifting to a uniform velocity by slowly and gradually controlling the acceleration as well as a method for simply performing linear acceleration.
That is, as disclosed in Jpn. Pat. Appln. KOKAI publication No. 2000-184142, Jpn. Pat. Appln. KOKAI publication No. 2000-307814 and the like, this is known as a control method for reducing the rising acceleration and decreasing a drive electric current to be supplied in a slow-up last period before rising to the constant velocity (uniform velocity) operation by which image reading scanning is performed.
However, even in this control method, control is performed by the same constantly high electric current value with any reading magnification during acceleration or during the constant velocity (uniform velocity) operation. Therefore, since the motor rotates with the excessive torque at a low velocity during acceleration with any reading magnification as a matter of course, vibrations are generated, which affects the scanning carriage.
As described above, when vibrations are generated in the scanning carriage, image vibrations are produced in a result of reading as a scanner due to the remaining vibrations.